The invention relates to a pressure sensor.
In pressure measurement technology, absolute- and relative-pressure sensors are used, for example. In the case of absolute-pressure sensors, a pressure to be measured is registered in absolute terms, i.e. as a pressure difference with respect to a vacuum. With a relative-pressure sensor, a pressure to be measured is picked up in the form of a pressure difference with respect to a reference pressure, for example a pressure which prevails where the sensor is located. In most applications, this is the atmospheric pressure at the place of use. Consequently, in the case of the absolute-pressure sensor a pressure to be measured is sensed in relation to a fixed reference pressure, the vacuum pressure, and in the case of the relative-pressure sensor a pressure to be measured is registered in relation to a variable reference pressure, for example ambient pressure.
Ceramic pressure-measuring cells are advantageously used in pressure measurement technology, since ceramic pressure-measuring cells have a measuring accuracy which is stable over a very long time. One reason for this is the solid ionic bonding of ceramic, which makes the material very durable and undergo virtually no ageing in comparison with other materials, for example metals. However, in comparison with metal, ceramic pressure sensors have a rougher surface and are often restrained by means of a generally nonreplaceble seal made of an organic material, for example an elastomer, in a pressure-tight manner in a housing which can then be fastened at a measuring location by means of a process connection.
In the food industry, pressure sensors which can be fitted such that they are flush at the front are used with preference, all sensor elements coming into contact with a medium of which the pressure is to be measured consisting of a metal, preferably of a stainless steel which can be cleaned very well.
In this branch of industry it is additionally of particular advantage if the pressure sensors have as few seals as possible. Seals consist of organic materials and, for reasons of hygiene, should therefore preferably be replaceable. In an ideal case, there is just a single seal for sealing off the process connection. In contrast to a seal belonging to the sensor, this seal, refereed to hereafter as the process seal, can be exchanged at any time by the user himself without any problem, in particular without any effect on the measuring accuracy of the pressure sensor.
It is an object of the invention to provide a pressure sensor which is suitable, inter alia, for the food industry and the measuring accuracy of which is stable over a long time.
For this purpose, the invention comprises a pressure sensor having
a diaphragm seal with a separating diaphragm, on which a pressure to be measured acts, and
a ceramic measuring cell, connected exclusively by inorganic materials to the diaphragm seal,
in which the separating diaphragm and all farther sensor components coming into contact during measurement with a medium where pressure is to be measured are metallic.
According to one development, the measuring cell is fixed without restraint in a housing by being seated in the axial direction on a small tube, via which the ceramic measuring cell is connected to the diaphragm seal.
According to one development, the measuring cell has a measuring diaphragm, which subdivides an interior space of the measuring cell into a first chamber and a second chamber. The first chamber is connected to the diaphragm seal via a small tube, the first chamber, the small tube and the diaphragm seal are filled with a fluid, the fluid transfers a pressure acting on the separating diaphragm to the measuring diaphragm, a reference pressure in the second chamber acts on the measuring diaphragm, and the pressure sensor has an electromechanical transducer for registering a deflection of the measuring diaphragm dependent on the pressure and the reference pressure and for converting said deflection into an electrical output signal.
According to one embodiment, the reference pressure is a reference pressure prevailing in the ambience and the second chamber has an opening through which the reference pressure is introduced into the second chamber, or the second chamber is hermetically sealed and the reference pressure is an absolute pressure prevailing in the second chamber.
According to one embodiment, the measuring cell is additionally enclosed in the radial direction in a holder.
According to one embodiment, the holder has a body made of an elastomer, filling an intermediate space between the measuring cell and the housing.
According to another development, the ceramic measuring cell is fastened in a housing connected to the diaphragm seal. In this case, the housing preferably consists of a material which has a coefficient of thermal expansion which is approximately equal to the coefficient of thermal expansion of the ceramic of the measuring cell.
According to one development, the measuring cell is fastened in an insert which is arranged in a housing, is connected to the diaphragm seal and reaches around the measuring cell in a pot-like manner. According to a further development of the invention, the measuring cell is mounted without restraint and isostatically in a chamber filled with a fluid, surrounded on all sides by the fluid.
Investigations have shown that, in the case of a ceramic measuring cell restrained in a pressure-tight conventional way by means of an organic material, for example a seal made of an elastomer, diaphragm seals cannot be used without sacrificing considerable measuring accuracy. Changes in temperature and/or pressure can cause positional and/or dimensional changes of the seal, which are accompanied by a displacement of diaphragm seal fluid. In the case of a diaphragm seal, only a small amount of the diaphragm seal fluid is displaced when there is a change in pressure. If there are seal-related volume displacements of the same order of magnitude as pressure-related volume displacements, meaningful measurement is no longer possible.
Customarily used sealing materials are plastics, such as polytetrafluoroethylene or Viton for example. These materials are not gastight. If a negative pressure acts on the pressure sensor, air or gas can diffuse into the diaphragm-seal fluid through the seal from a side of the pressure sensor facing away from the diaphragm seal. Air or gas in the diaphragm-seal fluid greatly impairs the measuring accuracy of the pressure sensor.
On account of the way in which the connection of the diaphragm seal is made according to the invention, using connections made of inorganic materials, seals can be dispensed with completely. It is consequently possible for the first time to use a ceramic pressure-measuring cell in connection with a diaphragm seal and to utilize the advantages of ceramic measuring cells, that is their stable measuring accuracy over a very long time, in connection with a diaphragm seal.
Mechanical connections of inorganic materials may be, for example, welded or soldered connections, in particular active brazed connections. Such metallic joints offer the advantage that they are gastight and, in comparison with methods of connection by means of organic materials, such as by means of restrained seals for example, are mechanically immovable and to the greatest extent free from creepage. Consequently, in the case of a pressure sensor according to the invention, changes in pressure and/or temperature do not cause permanent deformation of the connecting materials at the connection points that could lead to a deterioration in the measuring accuracy. The measuring accuracy of the pressure sensors according to the invention can therefore be guaranteed over very long time periods.
A further advantage is that, in spite of the use of a ceramic pressure-measuring cell, apart from the process seal, only metallic materials come into contact with the medium of which the pressure is to be measured. Use of the diaphragm seal makes it possible for the metal contacted by the medium to be freely selected within wide limits, according to the mechanical and/or chemical properties of the medium.
There are also applications in which the diaphragm seal is fitted directly at the measuring location in a so-called welded neck, i.e. a neck welded onto the container. In welded necks, the sealing usually takes place purely metallically, for example by means of sealing cones. In these cases, even the process seal is omitted.
The invention and further advantages are now explained in more detail with reference to the figures of the drawing, in which four exemplary embodiments are represented. The same elements are provided with the same reference numerals in the figures.